1. Field of the Invention
The invention relates in general to a trans-reflective organic electroluminescent panel and method of fabricating the same, and more particularly to the trans-reflective organic electroluminescent panel having an appropriate viewing angle, and improved luminescence efficiency and chromaticity.
2. Description of the Related Art
Use of organic electroluminescence device in the flat panel displays possesses several competitive advantages, such as self illumination, high brightness, wide viewing angle, vivid contrast, quick response, broad range of operating temperature, high luminous efficiency and uncomplicated process of fabrication. Thus, the organic electroluminescence device represents a promising technology for display applications and has receives the worldwide attention in the recent years.
The typical structure of organic electroluminescence device is mainly constructed by interposing an organic light emitting layer between an anode and a cathode. A hole transport layer (HTL) is interposed between the anode and the organic light emitting layer. An electron transport layer (ETL) is interposed between the cathode and the organic light emitting layer. Also, a hole injection layer (HIL) could be interposed between the anode and the hole transport layer. This laminated structure of OELD facilitates the electron flow from the cathode to the anode.
FIG. 1A illustrates a conventional non-microcavity organic electroluminescence device. The arrow of FIG. 1A denotes the light emitting direction. For the most structures of the organic electroluminescence devices, the anode 11 is made of the material with high transparency. Practically, a glass substrate 112 coated with a transparent and conductive indium tin oxide (ITO) layer 114 could function as the anode 11. The cathode 19 is made of the material with high reflectivity, such as the metallic stack including a lithium fluoride (LiF) layer and an aluminum (Al) layer. Also, a hole transport layer (HTL) 13, a light emitting layer 15 and an electron transport layer (ETL) 17 are formed orderly between the anode 11 and the cathode 19. However, the structure having the transparent and reflective electrodes, as shown in FIG. 1A, cannot generate the light disturbance inside the organic electroluminescence device (i.e. having no microcavity effect); the luminescence output and color saturation of the structure of FIG. 1A are inferior to that of the microcavity structure. Please refer FIG. 1B, which shows the emission spectra of the conventional organic electroluminescence device of FIG. 1A.
FIG. 2A illustrates a conventional microcavity organic electroluminescence device. The arrow of FIG. 2A denotes the light emitting direction. In a microcavity organic electroluminescence device of FIG. 2A, a semi-reflecting layer 21 is disposed between the glass substrate 112 and the ITO layer 114. In most the reported cases, the semi-reflecting layer 21 (as a light transmissive reflector) could be a quarter wave stack which is a multi-layer stack of alternating high index and low index dielectric thin films, each one a quarter wave-length thick. It can be turned to have high reflectance, low transmittance, and low absorption over a desired range of wavelengths. As shown in FIG. 2A, the semi-reflecting layer 21 comprises three titanium dioxide (TiO2) layers 211 and three silicon dioxide (SiO2) layers 213 (of the form TiO2:SiO2:TiO2:SiO2:TiO2:SiO2). Also, the cathode 19 of FIG. 2A is made of the material with high reflectivity. When the photons are emitted from the light emitting layer 15, some of them pass through the glass substrate 112 directly, and the others are reflected between the cathode 19 and the semi-reflecting layer 21. Since those reflective photons are interfered constructively or destructively, the light emission at a certain range of wavelength is greatly increased, and the light emission at the other range of wavelength is greatly decreased. FIG. 2B shows the emission spectra of the conventional microcavity organic electroluminescence device of FIG. 2A. The microcavity organic electroluminescence device indeed enhances the luminescence output and narrows the emission bandwidth (full-width-half-max, FWHM) when compared with the non-microcavity organic electroluminescence device (i.e. FIG. 1A). This microcavity effect also narrows the viewing angles of the organic electroluminescence device. In the top-emission device, the light interference becomes more serious because of the semi-transparent electrodes used as the anode and the cathode; thus, the microcavity effect is emphasized.
According to the description above, although the non-microcavity organic electroluminescence device presents a wider viewing angle (e.g. the device of FIG. 1B has the broader FWHM), it suffers from the lower luminescence output and unsaturated chromaticity. On the contrary, the microcavity organic electroluminescence device yields higher luminescence output and saturated chromaticity, but suffers from the very narrow viewing angle (e.g. the device of FIG. 2B presents the narrower FWHM) and color shift.